Field of the Invention
The present invention relates to a novel process for the preparation of (meth)acrylic acid which comprises a stage in which an aqueous solution which contains (meth)acrylic acid is extracted by being brought into contact with a solution which contains at least one starting material for the preparation of (meth)acrylic acid and forms a miscibility gap with the aqueous solution.
The term (meth)acrylic acid used in this Application refers to both acrylic acid and methacrylic acid.
Owing to its very reactive monoethylenically unsaturated bond and the acid function, methacrylic acid is a useful monomer for the preparation of polymers, for example for aqueous polymer dispersions suitable as adhesives.
Methacrylic acid is obtainable, inter alia, by gas-phase oxidation of 1-butene, isobutene, isobutyraldehyde, isobutyric acid, isobutene, MTBE and/or methacrolein with oxygen or oxygen-containing gases in the presence of catalysts, for example multimetal oxides which contain the elements molybdenum and vanadium in oxidic form. The oxidation is carried out at elevated temperatures and, owing to the considerable heat of reaction, preferably with dilution of the reactants with inert gases, such as N.sub.2, CO.sub.2 and/or hydrocarbons and/or steam. However, these processes do not give pure methacrylic acid but a reaction mixture which contains, in addition to methacrylic acid, the starting materials, for example unconverted methacrolein, steam, inert diluent gas (e.g. nitrogen) and byproducts (eg. oxides of carbon), lower aldehydes, eg. formaldehyde, high boilers, eg. citraconic acid, and in particular acetic acid, from which the methacrylic acid must subsequently be isolated (cf. for example EP-A 253 409 and DE-A 19 62 431). However, possible starting compounds are also those from which the actual C.sub.4 starting compound eg. methacrolein, is first formed as an intermediate during the gas-phase oxidation. An example is the methyl ether of tert-butanol (MTBE).
For the preparation of methacrolein, it is also possible to subject formaldehyde and propionaldehyde to a condensation reaction and to obtain methacrolein by distillation in higher purity than in the processes discussed above. Such a process is described in EP-B 58 927. The methacrolein thus obtained can then be converted into methacrylic acid in a conventional manner by catalytic gas-phase oxidation. Such a reaction is described, inter alia, in EP-B 297 445.
Acrylic acid can be obtained starting from the corresponding C.sub.3 compounds, in particular propylene and/or acrolein.
If, in a process for the preparation of (meth)acrylic acid, it is intended to isolate the latter from the resulting reaction gas mixture by extraction, the reaction gas mixture is first subjected to a condensation stage and then extracted. Thus, EP-B 345 083 describes such a process which comprises a methacrylic acid extraction stage in which methacrylic acid is extracted with a saturated hydrocarbon of 6 to 9 carbon atoms.
According to EP-A 710 643, in a process for the purification of methacrylic acid, an aqueous methacrylic acid solution obtained by cooling and condensing the reaction gas is extracted from the aqueous solution by adding an organic solvent, preferably an aliphatic hydrocarbon of 5 to 9 carbon atoms, an aromatic hydrocarbon, an ester or a mixture thereof.
Japanese Patent JP 57 095 938 describes the extraction of acrylic acid from a dilute aqueous solution by extraction by means of an oxygen-containing solvent and a tertiary amine. Examples are trioctylamine and 2,6-dimethyl-4-heptanol.
However, all these processes are disadvantageous in that in each case the extraction of (meth)acrylic acid requires the use of a foreign substance which, on the one hand, is not obtainable without additional costs and moreover, in such a process, necessitates additional steps for separating off this foreign substance again, entailing additional costs and additional energy consumption.
None of the prior art processes describes the possibility of extracting (meth)acrylic acid by means of a solution which contains an alkane, alkanol, alkene or alkenal of 3 or 4 carbon atoms or a mixture of two or more thereof, ie. a starting material for the preparation of (meth)acrylic acid. To date, only foreign substances, as stated above, have been used as extracting agents.
It is an object of the present invention to provide a process for the preparation of (meth)acrylic acid, by means of which the above disadvantages of the processes described to date for the preparation of (meth)acrylic acid can be wholly or partly avoided. A particular feature of the present invention is the extraction of (meth)acrylic acid without the use of foreign substances. Furthermore, this process should be capable of managing with a smaller amount of water and energy compared with the prior art processes.
We have found that these two objects too are achieved by the novel process, which, if desired, can be carried out entirely without distillation stages, which are known to be very energy-intensive.